Polyamide resins are widely used as engineering plastics having excellent mechanical strength such as impact resistance and friction/abrasion resistance as well as excellent heat resistance and oil resistance in the fields of automotive parts, electronic/electric equipment parts, office automation equipment parts, machine parts, construction materials/housing parts and the like.
Many classes of polyamide resins including e.g., polyamide 6 and polyamide 66 are known, among which m-xylylene adipamide (hereinafter sometimes referred to as “MXD6”) derived from m-xylylenediamine and adipic acid is positioned as a very excellent polyamide resin because it contains an aromatic ring in the main chain unlike polyamide 6, polyamide 66 and the like so that it has high rigidity, low water absorption and excellent oil resistance as well as a low molding shrinkage ratio and causes little sink marks or warpage, which means that it is also suitable for precision molding. For these reasons, MXD6 has recently been more widely used as a molding material, especially as an injection molding material in various fields including electronic/electric equipment parts, parts of vehicles such as automobiles, general machine parts, precision machine parts, leisure/sports goods, civil engineering and construction materials, etc.
With growing market demand for sophisticated and diverse products, lighter and stronger polyamide resin materials have also been needed and a known xylylenediamine polyamide resin lighter than MXD6 includes a xylylene sebacamide polyamide resin derived from xylylenediamine and sebacic acid (hereinafter sometimes referred to as “XD10”) (see patent document 1), which has been highly expected as a material for various parts especially in recent years because of its excellent chemical resistance and impact resistance.
However, MXD6 and XD10 crystallize more slowly than polyamide 6 and polyamide 66. Thus, it is difficult to allow MXD6 or XD10 alone to crystallize in a mold during injection molding so that it is very difficult to mold it into an article having a thin wall, and the resulting molded article is associated with problems including deformation and mechanical strength loss as well as great variation between lots of the molding in mechanical properties such as flexural strength, flexural modulus and impact resistance. Therefore, it was necessary to improve moldability by adding polyamide 66 having a high crystallization rate or a crystallization promoter such as talc powder to increase the crystallization rate or by increasing the mold temperature in order that MXD6 or XD10 could be used as a molding material (patent document 2). However, there was a limitation on the amount of the additives that could be incorporated because the incorporation of polyamide 66 invites greater property changes in a humid environment as compared with the case where MXD6 or XD10 is used alone or the incorporation of talc powder causes mechanical strength loss. Moreover, it was difficult to stably produce precision parts especially having a region of 1 mm or less in thickness by applying previously proposed MXD6 or XD10 because it did not homogeneously flow in a mold due to its crystallization rate and flowability as well as viscosity stability during the residence in the molten state and caused variation in the shape of the molded article or other problems.
Under these circumstances, there have been high demands for thin-wall articles that can be stably produced with little variation in mechanical properties such as flexural strength, flexural modulus and impact resistance.